Electron detectors

ABSTRACT

A cathode lens is formed between a gun electrode ( 8 ) and a specimen ( 9 ). An electron probe ( 11 ), produced as part of an electron column and suitably focused by lenses ( 1, 2  and  3 ) and scanned by suitable deflector/stigmator electrodes ( 2 ), is decelerated within the cathode lens field and its final landing energy is finely adjustable by the specimen negative bias. Emitted secondary electrons are re-accelerated within the same field and due to uniformity of this field, they increase their axial velocity only so that they are collimated into a narrow signal beam. The collimated signal beam passes mostly through an aperture ( 18 ) of electrode ( 8 ), where it enters the gun and a final lens consisting of a central earthed electrode ( 6 ) surrounded by two earthed electrodes ( 7 ) and ( 8 ). The signal beam approaches a special mirror electrode ( 4 ), the field of which decelerates and deflects the electrons further off the axis and returns them back towards the specimen ( 9 ). They are again re-accelerated in the final lens field, and impact a channel-plate electron multiplier ( 5 ) and after amplification, the signal electron beam impacts a collector ( 7 ), which can be divided into suitable parts for multichannel detection. The detector may have small dimensions and high resolution, and may be used in microscopes and other devices.

The present invention relates to electron detectors, and is concernedparticularly, although not exclusively, with electron detectors for usein electron microscopes.

It is known to provide a focused electron beam, referred to as anelectron probe, to impact onto a specimen under observation. The probeis scanned over a viewfield of adjustable dimensions and signalelectrons, emitted from the specimen and detected, are fed to a CRTmonitor scanned synchronously with the electron probe so that an imageis formed on the monitor screen, with the magnification given by ratioof sizes of both of the scanned fields.

From all possible signals excited by an electron probe, so-called Augerelectrons are of particular importance. They are released from atomswhich are originally ionised in an inner shell by an electron from theelectron probe; the vacancy is then filled by another electron from someother outer shell of the same or even different atom, and the energydifference is transmitted to one more electron which leaves the atomwith an energy characteristic to it. Consequently, when analysing theenergies of the signal electrons, one can recognise the surfaceelemental composition. By means of a finely focused electron probe, thiscan be done even at high spatial resolution. The energy analysis is madeby an electron spectrometer, i.e. some special configuration ofelectrostatic or magnetic fields or combination of fields that ensuresspatial dispersion of the electron trajectories according to theirenergy so that, by screening with suitable apertures, only a narrowenergy window can be filtered, or alternatively, parallel detection ofall emitted electrons is also possible.

Among possible spectrometer configurations utilising serial detection,the so-called Cylindrical Mirror Analyser (CMA) is popular and oftenused. It consists of two coaxial cylinders, the outer of which isnegatively biased so that electrons outgoing from a specified entrypoint on the cylinder axis through a suitable slit in the inner cylinderwall are back reflected through a second slit and a final smallaperture, again on the cylinder axis at an exit point, such that onlythose electrons which fall inside an energy window are passed.

In order to perform Scanning Auger Microscopy (SAM), i.e. to achieveelemental surface analysis with a high spatial resolution, an electrongun is co-axially positioned inside the inner cylinder of the CMA and isused for illuminating the specimen, placed on the device axisperpendicularly or inclined to it at the entry point, and has to be ahigh-quality electron gun with a very finely focused electron probe andwith facilities to scan the probe within the scanned field. Such asophisticated gun, the scanning column, is currently available equippedwith a field-emission single-crystal tip cathode and one or two electronlenses. It is of importance that the scanning column has to fit inside avery limited place surrounded by the spectrometer cylinder so that nomechanical actuators, lightpipes or similar connections can be led toit. All connections to the outside have to be exclusively electric andthe scanning column is not allowed to produce any electromagnetic fieldsin its vicinity, which would interfere with the spectrometer action.

The Auger electron is recognised according to a fixed value of itsenergy so that it can be detected only when it does not suffer from anyfurther scattering event causing an energy change. It means that onlyAuger electrons released within a few topmost atomic layers at thesurface of a specimen can contribute to the signal and the elementalcomposition of only such an ultrathin surface layer can be reliablymapped. Nevertheless, a typical electron probe will generally penetratemuch deeper into the specimen and a good number of excited electrons arereflected backwards from the surface with various energies, mostlysufficient to excite additional Auger electrons. In frequent cases, thespecimen is heterogeneous in its depth so that the back-reflectedelectrons vary locally, together with the quantity of the additionalAuger electrons, and the in-depth specimen heterogeneity projects itselfspuriously into the surface elemental mapping. In order to suppress thissignal contribution, some other depth sensitive signal has to beavailable. A second most important problem inherent to Auger mappings isthe contribution of the surface topography to the elemental mapping.Again, the mapping should be correlated with another microscopic signalshowing the surface relief.

There is currently a general trend in electron microscopy to lower theenergies of electrons in the electron probe. The reasons for thisinclude achievement of higher secondary electron signal (which has itsmaximum somewhere between a few hundred eV and a few keV); achievementof reduced charging of non-conductive specimens (owing to the totalyield of the emitted electrons approaching unity, so that only a smallproportion of them is dissipated inside the specimen); and betterresolution of tiny relief protrusions and ridges (owing to smallerinteraction volume and shorter penetration depth of the electron probe).Nevertheless, it is not possible to operate known probes below a fewhundreds of eV because of some principal obstacles, which includedeteriorated extraction of electrons from the gun cathode andconsequently lower electron probe current, longer electron wavelengthand higher relative fluctuations in energy of the electron probe (whichcauses larger spotsize due to increasing diffraction and chromaticaberrations) and more pronounced influence of spurious acelectromagnetic fields distorting the electron probe geometry. So calledlow-voltage microscopes working down to some 200 to 500 eV energy of theelectron probe are currently highly attractive and well marketed.Nevertheless, it is well known that below such an energy range, at tensand units of eV, many new extremely interesting contrasts appear whichvisualise the surface crystallographic structure, energy band structureabove the vacuum level, the potential barrier shape and its changes etc.

The only previously known way to realise very low energy microscopy, inthe range of tens and units of eV, is to use a cathode lens. The cathodelens is the crucial component of an emission electron microscope (EEM)in which the specimen itself emits the electrons and after necessaryacceleration they pass a projection electron-optical system forming themagnified image of the emitting surface. In principle, the cathode lensis an electrostatic lens consisting of two electrodes: the cathode, thespecimen surface itself, and a suitably shaped anode with a centralopening. The axial uniform electrostatic field between them, mostlyproduced by a high negative bias of the specimen/cathode, acts toaccelerate the emitted electrons. The non-uniform part of the field,penetrating through the anode opening, forms a diverging lens, which iscombined with some additional converging lens, the EEM objective lens.It is has been accepted for a long time that such a combination has verylow aberration coefficients so that even at very low energies, a broadbeam of the emitted electrons can be collimated into the imaging bundle.

Some attempts have been made during the last thirty years or so toutilise the cathode lens also in the reverse direction for decelerationof the electron probe immediately above the specimen surface.Nevertheless, none of these attempts has achieved significant successand no scanned very low energy pictures have been published. Animportant exception is the so called Low Energy Electron Microscope(LEEM) invented 35 years ago and successfully realised in the eighties[E. Bauer, Rep. Progr. Phys. 57(1994),895]. It is not a scanning devicebut an EEM with the specimen emission excited by the impact of acoherent planar electron wave. The cathode lens is passed twice, firstby the electron wave being decelerated and then by the emitted electronsin the opposite direction. The LEEM practice revealed the abovementioned attractive features of the very low energy range for surfacestudies. LEEM instruments, available in few laboratories in the worldonly, are large in size and comprise both electrostatic and magneticlenses essential for the detection of the LEEM signal. This is in favourof a scanning version of the LEEM (SLEEM) which is capable of producingsimilar results by much simpler apparatus.

In the area of SLEEM design and operation, important progress wasrecently made on the basis of improved theory of the cathode lens [M.Lenc, I. Müllerová, Ultramicroscopy 45(1992), 159] and a first series ofscanned micrographs were published exhibiting a consistent quality alongthe whole energy scale from a few tens of keV down to units of eV.Afterwards, even a method of adaptation of standard commercial ScanningElectron Microscopes (SEM) to the SLEEM method was elaborated [I.Müllerová, L. Frank, Scanning 15 (1993), 193]. In simplifieddescription, one can characterise this adaptation by insulation andbiasing of the specimen and introducing an anode above it: the mainproblem is then to tailor a detection system to the configuration.Nevertheless, the basic device is still a full SEM system with usualelectromagnetic lenses and coils.

The SLEEM signal, which brings specimen information with in-depthsensitivity similar to the surface sensitivity of SAM, represents theideal alternative for a complementary imaging device, necessary to solvethe crucial SAM problems described above. On the other hand, SLEEMimages of real heterogeneous, polycrystalline and similar specimens areoften filled with contrasts, straightforward interpretation of which isdifficult or even impossible without having further informationavailable, particularly those regarding the surface elementalcomposition as mediated by SAM. Thus, SAM and SLEEM are extremelysuitable to be combined in-situ in an ultrahigh-vacuum device, whichwould need to have the SLEEM column fulfilling the requirements put ontothe scanning column of the CMA based Auger microprobe, i.e. a miniaturepurely electrostatic SLEEM column with integrated detection system in acompact design.

No detection principle complying with these conditions has previouslybeen proposed, and preferred embodiments of the present invention aim toprovide devices which realise such a principle.

According to one aspect of the present invention, there is provided anelectron detector comprising:

an accelerator plate for accelerating electrons emitted from a specimen,the plate having an aperture through which said electrons pass;

a deflecting electrode arranged to deflect said electrons after passingthrough said aperture; and

a collector arranged to collect electrons deflected by said deflectingelectrode:

wherein said deflecting electrode is arranged to deflect said electronsby a process of secondary electron emission in response to saidelectrons impacting a deflecting surface of the deflecting electrode.

Preferably, said deflecting electrode is provided with an electronmultiplier material on said deflecting surface to deflect said electronsemitted from said specimen, such that said multiplier materialmultiplies such deflected electrons in use.

Preferably, said detector has a principal axis and said deflectingelectrode is arranged to deflect said electrons radially outwardly ofsaid principal axis.

Preferably, said deflecting electrode comprises a deflector plate formedwith an aperture which is of smaller diameter than that in saidaccelerator plate.

An electron detector as above may further comprise irradiating means forirradiating a specimen in order to cause emission of said electrons fromsaid specimen.

Preferably, said irradiating means is arranged to produce an irradiatingbeam that passes through said apertures in said accelerator anddeflector plates.

An electron detector as above may include means for focussing saidirradiating beam.

Preferably, said irradiating means comprises an electron gun.

Preferably, said accelerator plate and said specimen form a cathodelens.

An electron detector as above preferably has rotational symmetry aboutan axis of symmetry.

An electron detector as above preferably comprises means for applying anadjustable bias to the specimen.

The invention extends to an electron microscope provided with anelectron detector according to any of the preceding aspects of theinvention.

Preferably, said detector is mounted symmetrically on a principal axisof such a microscope.

For a better understanding of the invention, and to show how embodimentsof the same may be carried into effect, reference will now be made, byway of example, to the accompanying diagrammatic drawing, the singleFIGURE of which shows an example of an electron detector in accordancewith one embodiment of the invention, in a scanning low-energy electronmicroscope.

The illustrated electron detector is based on a multi-lensfield-emission electron column, the final lens of which is tailored toincorporate the detector. A cathode lens is formed between a gunelectrode 8 and a specimen 9. An electron probe 11, produced by theprobe forming part of the electron column and suitably focused by lenses1, 2 and 3 and scanned by suitable deflector/stigmator electrodes 2, isdecelerated within the cathode lens field and its final landing energyis finely adjustable by the specimen negative bias. Emitted secondaryelectrons are re-accelerated within the same field and due to uniformityof this field, they increase their axial velocity only so that they arecollimated into a narrow signal beam. The collimated signal beam can fitin its majority into an aperture 18 of electrode 8, where it enters thegun and a final lens consisting of a central earthed (or grounded)electrode 6 surrounded by two earthed (or grounded) electrodes 7 and 8.The signal beam approaches a special mirror electrode 4, the field ofwhich decelerates and deflects the electrons further off the axis andreturns them back towards the specimen. They are again re-accelerated inthe final lens field, and impact a channel-plate electron multiplier 5and after amplification, the signal electron beam impacts a collector 7,which can be divided into suitable parts for multichannel detection.

The FIGURE shows the final lens/detector part which is an importantaspect of this embodiment. This is attached to the probe 11 forming partof the column, which is in one version formed by a field-emission tipcathode based gun 12 and one electrostatic lens, to produce apre-focused electron probe. The lens/detector part consists of theearthed (or grounded) electrode 1 as the first element of the final lensand the set of stigmator/deflector plates 2. The latter are arrangedaround the optical axis (they are eight in one version) and biased bycomposite ac and dc voltages which are adjusted so that they deflectstatically the electron probe to align it. They form a cylindrical lensrotatable around the axis (in order to correct for the axial astigmatismof the column) and finally they scan the electron probe across thescanned field.

Further along the optical axis, there is the central, negatively biasedfocusing electrode 3 of the final lens, the bias of which governs thefocal length and enables one to focus the electron probe just onto thesurface of the specimen 9. It is followed by the most important part ofthe detector, the mirror electrode 4. The shape of this is important andconsists of a small central bore 14, several tens of mm in diameter inone version, a cone 24 inclined to the horizontal (as seen) by an angleθ and a peripheral elevated rim 34. The angle θ is important for thefunction of the mirror electrode 4 but its value varies in a very broadinterval, virtually over the whole range from 0 to π/2, according tovoltages used and according to the column working distance. The mirrorelectrode 4 is also negatively biased to an adjustable potential.

The field above the mirror surface acts in such a way that itdecelerates the incoming electrons and its radial component deflects themajority of them off the axis, except those moving exactly along theoptical axis. Thus, the electrons reach sufficient distances from theaxis before they impact the mirror surface. A proportion of these signalelectrons “slip” the central cone and impact onto the peripheral rim orto somewhere in between. Thus, the mirror electrode is intensivelybombarded by electrons, the energy of which depends on the voltagesused. Under this bombardment, the mirror surface emits secondary (or, inthis example, tertiary) signal electrons 10 with energies mostly a feweV only. In one version, the secondary (tertiary) electron yield isenhanced by means of the mirror electrode surface being coated bysuitable material with high secondary (tertiary) electron yield. Theslow secondary (tertiary) electrons 10 are then again acceleratedtowards the upper surface of the channel-plate electron multiplier 5(e.g. a plate consisting of sintered very thin glass tubes, slightlyinclined with respect to the plate normal, coated on their inner wallsby a material with very high secondary electron yield).

For its function, the channel-plate electron multiplier (CP) needs apotential difference to be applied between its surfaces so that theentry surface is negatively biased with respect to the exit surface.Nevertheless, the potential of the upper entry surface is less negativethan that of the mirror 4, so that the slow electrons 10 emitted fromthe mirror electrode 4 are accelerated and due to radial field componentabove the conical part of the mirror 4, they are further deflected offthe axis but simultaneously, the influence of the peripheral rim of themirror 4 causes the electrons not to exceed the optimum distance off theaxis so that they in large majority impact the CP 5.

The electrode 6 bearing the collector 7 follows the CP 5. The electrode6 is the second earthed electrode of the final lens and it is shaped insuch a way that it screens, in one version by a thin-wall coaxial tube,the optical axis from the field due to the CP potential difference, sothat its influence on the electron probe is minimised. The collector 7is made from an insulating material on which a structure of segments isarranged in order to sort the detected signal according to azimuthalangles or radial distances of the detected electrons. Both the collector7 and the electrode 6 are held at earth potential but the exit bottomsurface of the CP 5 is at some low negative potential.

The whole assembly is closed with the electrode 8 serving as a columncup with a central bore. The electrode 8 is again earthed and it servesas the anode of the cathode lens, the cathode of which is formed by thespecimen 9 itself, connected to a stable finely adjustable high-voltagesupply of negative potential, at least slightly exceeding the nominalgun voltage (in order to provide for the mirror imaging, necessary forthe adjustment).

In one version, the whole assembly is embedded into a gun cap, which hasa conical shape which fits the inner shape of a hollow conical electronbeam accepted by a CMA 13. The individual elements are mutuallyinsulated, appropriately, with respect to their biases. The insulatinginserts are in one version made from a machineable glass ceramicmaterial, having elements which are mechanically pre-centred andelectrically connected to connector pins at the bottom end of the gun,so that the whole column is simply plugged into a socket fixed to theCMA inner cylinder opposite to the specimen, but away from the spacetraversed by the exit electron beam of the CMA analyser.

The device is capable of functioning even without any bias applied tothe specimen 9. In this case, backscattered electrons (BSE) are detectedas the signal electrons, namely those which fit into the space anglelimited by the central opening in the electrode 8. After passing thisopening inwardly of the gun, they behave identically as there-accelerated signal electrons in the SLEEM mode, as described above.The BSE imaging signal is significantly lower than the SLEEM imagingsignal, due to the small acceptance angle.

The above-described method and device fulfils completely therequirements for a scanning column suitable to work inside a CMAspectrometer or similar device producing SAM mappings, such that it ispurely electrostatic, of very compact design, not producing any spuriouselectromagnetic fields in its vicinity, and efficient in excitation anddetection of signal electrons in SLEEM mode, with an electron probeoperating at energies ranging from several kilo electron volts down tozero impact energy at which the probe is reflected immediately above thespecimen surface. The device may operate additionally in BSE mode with anon-biased specimen. This gives the possibility of a very low energyelectron microscope device.

Although the illustrated embodiment is for use with an electronmicroscope, alternative embodiments of the invention may have otherapplications. Means other than an electron gun may be utilised tostimulate emission of electrons from a specimen.

Although it is preferred that, as in the illustrated example, thespecimen 9 is excited by an irradiating beam that passes along theoptical axis of the apparatus, alternative means of exciting thespecimen may be employed. For example, the specimen may be irradiated byone or more beam that is incident to its surface at angles of less than90°.

A detector as illustrated has been found to detect electrons with anefficiency ranging from 80% for 1 eV emitted electrons, 97% for 10 eVemitted electrons, and 20% for 100 eV emitted electrons, to 2% for 1000eV emitted electrons. By using low electron energy, column resolutionhas been found to improve over previously proposed devices by two ordersof magnitude or more—e.g. from 5000 nm to 50 nm in one embodiment havingCs˜150 mm and Cc˜50 mm.

A detector in accordance with embodiments of the invention may besuitable for UHV applications; surface analysis applications whencombined with, for example, a CMA; general SEM applications, especiallywhen the final lens is electrostatic or compound electrostatic withmagnetic; and radiation sensitive material/sample applications. Anotherapplication is to the imaging of non-conductive specimens, such as thesurfaces of photoresist materials, such that high resolution can beobtained for non-charging imaging of non-conducting surfaces such asresists at suitably selected energies.

The term “earth potential” (or like terms such as “ground” potential orvoltage) is used conveniently in this specification to denote areference potential. As will be understood by those skilled in the art,although such reference potential may typically be zero potential, it isnot essential that it is so, and may be a reference potential other thanzero.

In this specification, terms of absolute orientation are usedconveniently to denote the usual orientation of items in normal useand/or as shown in the accompanying drawings. However, such items couldbe disposed in other orientations, and in the context of thisspecification, terms of absolute orientation, such as “top”, “bottom”,“left”, “right”, “vertical” or “horizontal”, etc. are to be construedaccordingly, to include such alternative orientations.

In this specification, the verb “comprise” has its normal dictionarymeaning, to denote non-exclusive inclusion. That is, use of the word“comprise” (or any of its derivatives) to include one feature or more,does not exclude the possibility of also including further features.

What is claimed is:
 1. An electron detector having a principal axis andcomprising: an accelerator plate for accelerating electrons emitted froma specimen, the plate having an aperture through which said electronspass; a deflecting electrode arranged to deflect said electrons afterpassing through said aperture, by a process of secondary electronemission in response to said electrons impacting a deflecting surface ofthe deflecting electrode; and a collector disposed between said specimenand said deflecting electrode and arranged to collect electronsdeflected by said deflecting electrode: wherein said deflecting surfaceof said deflecting electrode is inclined at an obtuse angle to saidprincipal axis such that electrons emitted from the surface of thespecimen travel outwardly from said principal axis and towards saiddeflecting surface to generate further secondary and tertiary electronswhich in turn travel towards said collector, and the electron detectorhas rotational symmetry about said principal axis.
 2. An electrondetector according to claim 1, wherein said deflecting electrode isprovided with an electron multiplier material on said deflecting surfaceto deflect said electrons emitted from said specimen, such that saidmultiplier multiplies such deflected electrons in use.
 3. An electrondetector according to claim 1, wherein said deflecting electrode isarranged to deflect said electrons radially outwardly of said principalaxis.
 4. An electron detector according to claim 1, wherein saiddeflecting electrode comprises a deflector plate formed with an aperturewhich is of smaller diameter than that in said accelerator plate.
 5. Anelectron detector according to claim 1, further comprising irradiatingmeans for irradiating a specimen in order to cause emission of saidelectrons from said specimen.
 6. An electron detector according to claim5, wherein said deflecting electrode comprises a deflector plate formedwith an aperture which is of smaller diameter than that in saidaccelerator plate and said irradiating means is arranged to produce anirradiating beam that passes through said apertures in said acceleratorand deflector plates.
 7. An electron detector according to claim 5,including means for focussing said irradiating beam.
 8. An electrondetector according to claim 5, wherein said irradiating means comprisesan electron gun.
 9. An electron detector according to claim 8, whereinsaid accelerator plate and said specimen form a cathode lens.
 10. Anelectron detector according to claim 1, further comprising means forapplying an adjustable bias to the specimen.
 11. An electron microscopeprovided with an electron detector according to claim
 1. 12. An electronmicroscope according to claim 11, wherein said detector is mountedsymmetrically on a principal axis of the microscope.